1. Field of the Invention
This invention relates to gas detectors, and more particularly to gas detectors utilizing semiconductor gas sensing elements.
2. Description of the Prior Art
In order to detect dangerous gases such as a flammable gas and a poisonous gas contained in an atmosphere, various kinds of semiconductor gas detectors have been proposed and developed. FIG. 1 illustrates one example of typical conventional semiconductor gas detectors, which comprises a pair of input terminals 1 and 2 to be connected to a d.c. power source (not shown), a series combination of a resistor R and a semiconductor gas sensing element A connected across the input terminals 1 and 2, and a pair of output terminals 3 and 4 connected across the resistor R. Although not illustrated, the output terminals 3 and 4 are connected to a suitable means for indicating or recording the output voltage.
The semiconductor gas sensing element A is of one of the various kinds of well-known n-type reducing oxide semiconductor materials such as n-type tin oxide containing a small amount of a suitable catalyzer. The electrical resistance-to-gas characteristic of the sensing element, and, accordingly, the output voltage characteristic to gas of the gas detector in which the element is used, can be easily varied to a desirable characteristic only by changing the amount of the catalyzer added in the semiconductor material. The gas detector element generally has any suitable heating means h for heating the sensing element to an elevated temperature for activation.
The gas detector illustrated in FIG. 1 exhibits the voltage-to-temperature characteristics across the output terminals 3 and 4 as shown by the several curves in FIG. 2. In the graph, the output voltage of the gas detector is plotted against various temperatures in centigrade. Curves P, H, M and I show the output voltages across the output terminals 3 and 4 when the gas detector is placed in an atmosphere of air containing a 1,000 ppm gaseous component of propane gas, hydrogen gas, methane gas and isobutane gas, respectively, with a voltage of 100V applied across the input terminals 1 and 2 and a resistance of 4 k.OMEGA. selected for the resistor R.
When, instead of the first gas sensing element A, another different second gas sensing element A' which contains a different amount of the catalyzer previously described is employed in the same gas detector circuit as illustrated in FIG. 1, the gas detector with the second gas sensing element A' exhibits output voltage-to-temperature characteristics for propane gas, hydrogen gas, methane gas and isobutane gas as shown in FIG. 3, which is quite different from that shown in FIG. 2. The characteristic curve P, H, M and I are those obtained under the same conditions as those in FIG. 2 except for the second gas sensing element A'.
From these characteristic curves shown in FIGS. 2 and 3, it is easily seen that with either one of the first and second sensing elements A and A', plural kinds of gaseous components are detected at the same time, thereby prohibiting the gas detector from selectively detecting a particular gaseous component to be detected. In some cases, it is impossible to detect dangerous gases such as a flammable gas and a poisonous gas through the use of the conventional gas detector employing a single semiconductor gas sensing element. Thus, the conventional gas detector is unsatisfactory or, in some cases, useless for those purposes. Also, the conventional gas detector is unable to identify the gaseous component detected and is unreliable and ineffective.